Assessing tissue contact with catheter using pairs of electrodes and common reference ground established using designed circuit-board capacitance

ABSTRACT

An apparatus includes a current source, an electronic circuit and a circuit board. The current source is configured to flow an electrical current having a selected frequency between a pair of electrodes coupled to a medical probe. The electronic circuit is configured to measure a single-ended voltage relative to ground that is formed on at least one of the electrodes in the pair in response to the electrical current, and, based on the measured voltage, to assess physical contact between the at least one of the electrodes and tissue. The circuit board includes the current source and the electronic circuit, and includes a layout that produces, at the selected frequency, a predefined capacitance between the current source and ground, thus forming a reference for measurement of the single-ended voltage.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical devices, andparticularly to methods and systems for assessing contact betweencatheter and tissue.

BACKGROUND OF THE INVENTION

In various medical procedures, such as in ablation of heart tissue, aphysician needs to know the extent of physical contact between a medicalprobe and tissue when performing a medical procedure to the tissue.Various techniques for determining proximity and contact are known inthe art.

For example, U.S. Patent Application Publication 2016/0143686 describesa method of determining the distance between an electrode catheterdisposed in a body fluid adjacent an internal body surface, and theinternal body surface, the method includes applying an alternatingvoltage or an alternating current, determining the impedance between atleast one pair of electrodes on the electrode catheter, and determiningthe distance between the electrode catheter and the internal bodysurface based at least in part on the determined impedance.

U.S. Patent Application Publication 2016/0287137 describes a method andsystem for assessing electrode-tissue contact before the delivery ofablation energy. The method includes determining a difference between amaximum impedance magnitude at a low frequency for a given electrode andan absolute minimum impedance magnitude at the low frequency across allelectrodes. The method further includes determining a difference betweena maximum impedance magnitude at a high frequency for a given electrodeand an absolute minimum impedance magnitude at the high frequency acrossall electrodes, and determining a difference between a maximum impedancephase at the high frequency for a given electrode and an absoluteminimum impedance phase at the high frequency across all electrodes.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesan apparatus that includes a current source, an electronic circuit and acircuit board. The current source is configured to flow an electricalcurrent having a selected frequency between a pair of electrodes coupledto a medical probe. The electronic circuit is configured to measure asingle-ended voltage relative to ground that is formed on at least oneof the electrodes in the pair in response to the electrical current,and, based on the measured voltage, to assess physical contact betweenthe at least one of the electrodes and tissue. The circuit boardincludes the current source and the electronic circuit, and includes alayout that produces, at the selected frequency, a predefinedcapacitance between the current source and ground, thus forming areference for measurement of the single-ended voltage.

In some embodiments, the electronic circuit includes an applicationspecific integrated circuit (ASIC). In other embodiments, the electrodesof the pair are fixed at opposite ends of a first section of the medicalprobe, and include an additional current source, which is configured toflow the electrical current between an additional pair of electrodescoupled to opposite ends of a second section of the medical probe,different from the first section, each of the additional electrodes iselectrically coupled to the electronic circuit so as to assess physicalcontact between at least one of the additional electrodes and tissue. Inyet other embodiments, the apparatus includes a processor, which isconfigured, based on the assessed physical contacts, to output whetherthere is physical contact between the tissue and at least one of (i) theelectrodes, and (ii) the additional electrodes.

In an embodiment, the first and second sections are not overlapped withone another. In another embodiment, the electronic circuit is configuredto indicate that the at least one of the electrodes is in physicalcontact with the tissue when the measured voltage is above a predefinedthreshold, and to indicate that the at least one of the electrodes isnot in physical contact with the tissue when the measured voltage isbelow the predefined threshold.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method including flowing from a current source anelectrical current having a selected frequency between a pair ofelectrodes coupled to a medical probe. Using an electronic circuit, asingle-ended voltage is measured relative to ground that is formed on atleast one of the electrodes in the pair in response to the electricalcurrent, and, based on the measured voltage, physical contact betweenthe at least one of the electrodes and tissue is assessed. A circuitboard, which includes the current source and the electronic circuitincludes a layout that produces, at the selected frequency, a predefinedcapacitance between the current source and ground, thus forming areference for measurement of the single-ended voltage.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, pictorial illustration of a system for ablatingtissue of a patient heart, in accordance with an embodiment of thepresent invention; and

FIG. 2 is a schematic, pictorial illustration of a module for assessingcontact between catheter and tissue, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Medical probes, also referred to herein as catheters, are used in avariety of medical procedures, such as in radio-frequency (RF) ablationof heart tissue. When performing an ablation procedure, it is importantto ensure physical contact between ablation electrodes of the probedistal tip and the target tissue, so as to form the desired lesion inthe tissue and to prevent the formation of byproduct blood clots thatmay put the patient at risk.

Embodiments of the present invention that are described hereinbelowprovide improved techniques for assessing physical contact between agiven section of a catheter distal tip and target tissue, e.g., inperforming electro-potential (EP) mapping of patient heart, and/or inablation to the patient heart and/or any other procedure involvingphysical contact between a catheter distal tip and target tissue.

The distal tip typically comprises one or more pairs of electrodescoupled to the distal tip along the given section. In some embodiments,each pair of electrodes is coupled to a respective current source, whichis configured to flow an electrical current having a selected frequencybetween the electrodes of the pair. The electrodes of each pair areelectrically isolated from one another, so that electrical current canflow between the electrodes only when some conductive material such astissue or blood is in contact with both electrodes.

In some embodiments, the current sources are mounted on a circuit board(CB). The CB further comprises an electronic circuit, such as anapplication specific integrated circuit (ASIC), which is configured tomeasure a single-ended voltage between each electrode and ground.

In order to enable a meaningful measurement of the voltage on anelectrode (i.e., a voltage that is truly a known function of the currentflowing through the electrode), the electrode and the current sourceshould be referenced to a common reference ground.

In some embodiments of the present invention, a common reference groundfor the electrodes and the current sources is established using thelayout of the CB on which they are mounted. For this purpose, the CBlayout is designed to have, at selected frequencies of the currentsources, predefined capacitances between the current sources and ground.

For a given electrode, belonging to a given electrode pair, the unipolarvoltage develops on the electrode in response to the electrical currentflowed through the pair by the respective current source. The predefinedcapacitance of the layout references the current source to ground,rather than being floating, thereby enabling a meaningful measurement ofthe unipolar voltage relative to ground potential.

With the common reference ground established, the measured unipolarvoltage is indicative of a respective impedance of the tissue or bloodthat is in physical contact with the pair of electrodes. Since the bloodhas better electrical conductivity than the tissue, the impedance andtherefore the voltage measured between the electrodes is higher when incontact with the tissue than with the blood alone. In some embodiments,based on the measured unipolar voltages, the ASIC is configured toassess whether there is a physical contact between the heart tissue andthe given section of the distal tip.

The disclosed techniques improve patient safety in ablation proceduresbut are not limited to ablation, and can be used, for example, in EPmapping and other procedures. Furthermore, the disclosed techniquesincrease the functionality and reduce the cost of an ablation catheterby integrating a tissue proximity indicator (TPI) with animpedance-based position tracking system of the catheter.

System Description

FIG. 1 is a schematic, pictorial illustration of a system 10 forablating tissue of a patient heart 40, in accordance with an embodimentof the present invention. In some embodiments, system 10 supportsconstruction of a mapping of heart 40 of a patient 14, and using theconstructed mapping for navigating a medical tool within heart 40,during an ablation procedure.

Reference is now made to an inset 25. In some embodiments, system 10comprises a medical probe, such as a catheter 12, having a distal tip13. In some embodiments, distal tip 13 comprises a plurality of devices,such as a pair of electrodes 52 and 54 coupled to respective ends of agiven section of the distal tip. In some embodiments, electrodes 52 and54 may have various roles in distal tip 13. For example, electrodes 52and 54 may serve as impedance-based position sensors, and/or aselectro-potential (EP) sensing electrodes, and/or as ablationelectrodes. In this configuration, distal tip 13 of catheter 12 may beused for mapping and/or for ablating tissue 33 of heart 40.

During the mapping phase a physician 16 may insert catheter 12, via aninsertion point 30, into vasculature of patient 14, and may thennavigate the catheter tip to heart 40. Subsequently, catheter 12 is usedfor mapping tissue 33 of heart 40 before ablating the tissue.

In the context of the present invention and in the claims, the term“electrical impedance” is also referred to herein simply as “impedance”for brevity.

In some embodiments, an operating console 18 comprises a radiofrequency(RF) generator 22, configured to generate the RF ablation signalsapplied by catheter 12 to tissue 33 of heart 40.

In some embodiments, console 18 comprises a processor 20, typically ageneral-purpose computer, with suitable front end and interface circuitsfor receiving signals from catheter 12 and for controlling othercomponents of system 10 described herein. Processor 20 may be programmedin software to carry out the functions that are used by the system, andthe processor stores data for the software in a memory 21. The softwaremay be downloaded to console 18 in electronic form, over a network, forexample, or it may be provided on non-transitory tangible media, such asoptical, magnetic or electronic memory media. Alternatively, some or allof the functions of processor 20 may be carried out by dedicated orprogrammable digital hardware components.

In some embodiments, system 10 further comprises an impedance-basedactive current location (ACL) system, typically used for tracking theposition of distal tip 13 for the purpose of navigating catheter 12 toablation locations within heart 40 of patient 14.

In an embodiment, the position of distal tip 13 is shown on an image 42of heart 40, which is displayed on a user display 34. In someembodiments, image 42 is acquired using an anatomical imaging system,such as a computerized tomography (CT) system or any other suitableimaging technique.

In some embodiments, the ACL system comprises a plurality of electrodes28, which are coupled to the body of patient 14, e.g., via patches 29that adhere to the skin of patient 14. In the example of FIG. 1 , system10 comprises six electrodes, of which electrodes 28 a, 28 b, and 28 care coupled to the front (e.g., chest) of patient 14, and electrodes 28d, 28 e, and 28 f are coupled to the back of patient 14. In otherembodiments, system 10 may comprise any suitable number of electrodes,coupled to the patient skin in any suitable arrangement.

Electrodes 28 of patches 29 are typically connected, via a cable 32, toprocessor 20, which is configured to receive, from electrodes 28 andfrom other sensors information such as values of impedance. Based onthis information, processor 20 is configured to estimate the position ofdistal tip 13 within heart 40.

Display 34, is typically configured to facilitate performance of themapping and/or ablation procedures by displaying relevant information tophysician 16. For example, processor 20 may register between thecoordinate systems of an impedance-based position tracking system andthe coordinate system of the CT system (which acquired image 42), so asto display the location and orientation of distal tip 13 within image42.

As noted above, electrodes 28 are typically used for navigating catheter12 within the body of patient 14, using impedance-based trackingtechniques, such as those described, for example, in U.S. Pat. No.8,456,182 and US Patent Application Publication 2015/0141798, whosedisclosures are incorporated herein by reference. Such techniquesinvolve estimating the location and orientation of distal tip 13responsively to the different impedances measured between distal tip 13and each of electrodes 28 a-28 f. As described above, the estimatedlocation of distal tip 13 may be indicated to the physician as asuitable icon on display 34. Based on this indication, physician 16 maynavigate distal tip 13 of catheter 12 to one or more target locationswithin heart 40, and subsequently ablate tissue at one or more of thetarget locations.

In some embodiments, the location and orientation of distal tip 13 atany given time, are typically estimated by applying an electrical signalof a known amplitude to distal tip 13, and the resulting voltagegradients and/or currents are measured at each pair of electrodes 28. Inalternative embodiments, the electrical signal may be applied byelectrodes 28, and the resulting electrical values are measured by oneor more sensors of distal tip 13.

In some embodiments, these applied electrical signals cause electrodes28, each of which is located at a different position relative to thecatheter, to exhibit different respective electrical values, due to adifferent amount of electrically-impeding tissue (and therefore, adifferent degree of impedance) between distal tip 13 and each electrodeamong electrodes 28.

In some embodiments, these measured electrical values are sent, viacable 32, to processor 20, which uses these values to estimate therelative location and orientation of distal tip 13 relative toelectrodes 28 (whose positions are known). Alternatively, voltagegradients between the distal tip of the catheter and the electrodes maybe generated, and the resulting currents flowing through the electrodesmay be measured and used for estimating the location and orientation ofdistal tip 13.

In some embodiments, in a calibration procedure referred to herein as“mapping,”, processor 20 is configured to construct a set of data pointsthat each comprises the position and electrical values measured at arespective position visited by distal tip 13. In an embodiment, whencompleted, the mapping is applied (e.g., during ablation) to electricalvalues acquired by distal tip 13 and/or electrodes 28, for translatingmeasured electrical values into a position measurement in heart 40.

Note that a separate mapping may be constructed for selected respirationoperations (for example, after a full inhalation operation, after a fullexhalation operation, or a midpoint between inhalation and exhalationoperations) of patient 14.

Reference is now made again to inset 25. Before ablating tissue 33, itis important to bring the ablation electrodes of distal tip 13 inphysical contact with tissue 33. In some embodiments, system 10 isconfigured to flow an electrical current to electrodes 52 and 54. Inresponse to the electrical current, a voltage develops between theelectrodes, wherein the voltage value depends on whether the electrodesare in contact with the tissue or blood. The ASIC measures for at leastone of electrodes 52 and 54 a single-ended voltage, also referred toherein as a unipolar voltage, which is indicative of a physical contactbetween the given section of distal tip 13 and tissue 33. Furtherdetails on the configuration of system 10, which is related to thetissue contact sensing are described in FIG. 2 below

As shown in inset 25, distal tip 13 is in physical contact with tissue33. In this position, electrical current flows through electrical path15 between electrodes and 54 via tissue 33. When both electrodes are inphysical contact with the tissue, this voltage is indicative of anelectrical impedance of the tissue, also referred to herein as Z-tissue.

Similarly, when distal tip 13 (i.e., at least one of the electrodes) isnot in physical contact with tissue 33, the electrical current flowsbetween electrodes 52 and 54 via blood 43. In this position, theunipolar voltage on at least one of electrodes 52 and 54 (relative tothe ground), is indicative of an electrical impedance of the blood, alsoreferred to herein as Z-blood, which is typically significantly smallerthan the electrical impedance of tissue 33. In some embodiments,processor 20 or any other processing device, as will be shown in FIG. 2below, is configured to measure the unipolar voltage and based on themeasured voltage, and to assess whether there is physical contact in thesection between the respective electrodes (e.g., electrode 52 or 54) ofdistal tip 13 and tissue 33.

A Module for Assessing Contact Between Probe and Tissue Using One orMore Pairs of Electrodes

FIG. 2 is a schematic, pictorial illustration of a module 44 forassessing contact between distal tip 23 and tissue 33, in accordancewith an embodiment of the present invention. Distal tip 23 may replace,for example, distal tip 13 of catheter 12 shown in FIG. 1 above.

In some embodiments, module 44 comprises a circuit board (CB) 66, whichis electrically coupled, via electrical traces 64, and leads 65 runningthrough the catheter, to electrodes 52, 54, 56 and 58 of distal tip 23.In some embodiments, CB 66 comprises a current source 55, which isconfigured to flow, via electrodes 52 and 54 of distal tip 23, traces 64and leads 65, an alternating electrical current having a selectedfrequency (e.g., 6.3 kHz). As described in FIG. 1 above, the electricalcurrent flows between electrodes 52 and 54 through any electrical path,such as path 15, which is electrically connecting therebetween. In thepresent example, the electrical current may flow through blood 43 orthrough tissue 33, each of which has a different impedance.

In some embodiments, current source 55 is electrically connected, viathe layout of CB 66, to a ground 62. Note that by design, the layout ofCB 66 creates a predefined capacitance, at the selected frequency ofcurrent source 55, between the current source and ground 62. Thiscapacitance is shown schematically as a capacitor 71. In thisconfiguration, current source 55 is not floating. Rather, at theselected frequency of the alternating current generated by currentsource 55, the predefined capacitance is configured to establish ground62 as a reference ground for current source 55.

In some embodiments, current source 55 is configured to produce aconstant electrical current between electrodes 52 and 54, so that thecurrent value does not depend on the impedance between the electrodes.Thus, a respective single-ended voltage is formed between each electrodeamong electrodes 52 and 54, and ground 62. Note that the voltage on eachof electrodes 52 and 54 is indicative of the impedance betweenelectrodes 52 and 54, and therefore is indicative of whether electrodes52 and 54 are in contact with tissue 33 or with blood 43.

In some embodiments, module 44 further comprises an application specificintegrated circuit (ASIC) 60, which is mounted on CB 66 and iselectrically coupled to electrodes 52 and 54. In the example of FIG. 2 ,CB 66 comprises traces 68 that electrically connect, via traces 64 andleads 65, between ASIC 60 and each electrode among electrodes 52 and 54.In other embodiments, module 44 may comprise, in addition to or insteadof ASIC 60, any other suitable electronic circuit, which is electricallycoupled to electrodes 52 and 54 using any suitable interconnectingconfiguration.

In some embodiments, ASIC 60 may comprise any suitable IC device, suchas a general-purpose processor, which is programmed in software to carryout the functions described herein. The software may be downloaded tothe processor in electronic form, over a network, for example, or itmay, alternatively or additionally, be provided and/or stored onnon-transitory tangible media, such as magnetic, optical, or electronicmemory.

In some embodiments, ASIC 60 is configured to measure the single-endedvoltage relative to ground that is formed on at least one of electrodes52 and 54, in response to the electrical current of source current 55.As described above, the measured voltage is indicative of the impedanceof the electrical path between electrodes 52 and 54. In the example ofFIG. 1 , the electrical path between the electrodes may pass throughtissue 33, blood 43, or through a combination thereof. Thus, based onthe measured voltage, ASIC 60 is configured to assess the physicalcontact between tissue 33 and the one or more respective electrodes ofdistal tip 13. Note that in the configuration of FIG. 2 , the layout ofCB 66 is designed to form a reference ground for measurement of thesingle-ended voltage.

In some embodiments, the single-ended voltage that is formed, forexample, on electrode 54 relative to ground 62, provides an indicationof whether distal tip 13 is in physical contact with tissue 33 in asection located between electrodes 52 and 54.

In some embodiments, distal tip 13 may comprise additional pairs ofelectrodes, such as electrodes 56 and 58, located at the ends of anothersection along distal tip 13. Typically, although not necessarily, thesection between electrodes 56 and 58 does not overlap with the sectionlocated between electrodes 52 and 54. In these embodiments, a currentsource 57, which is similar to current source 55, is coupled toelectrodes 56 and 58 and is configured to flow an alternating electricalcurrent. Typically, although not necessarily, both current sources 55and 57 (or all current sensors in the case of more than two pairs ofelectrodes) have the same frequency. In response to the electricalcurrent, the single-ended voltage relative to ground 62 that is formedon at least one of electrodes 56 and 58, is measured by ASIC 60 asdescribed above.

In an embodiment, the layout of CB 66 is designed in to provide, at theselected frequency, a predefined value of capacitance between currentsource 57 and ground 62, shown schematically as a capacitor 73.

In another embodiment, current source 57 may be configured to flow anelectrical current having a given frequency, different from the selectedfrequency of current source 55. In this embodiment, the layout of CB 66is configured to create, at the given frequency, a given capacitancebetween current source 57 and ground 62. Note that the given capacitancediffers from the predefined capacitance and configured to form a commonreference ground for measurement of the single-ended voltage thatcorresponds to the given frequency. In this embodiment, using thetechniques described above, ASIC 60 is configured to assess physicalcontact between tissue 33 and the section of distal tip 23 locatedbetween electrodes 56 and 58.

By distributing along distal tip 13 additional pairs of electrodes, suchas electrodes 52 and 54 coupled to respective electronic circuitries asdescribed above, module 44 is configured to detect physical contactbetween each of the respective pairs and tissue 33. In variousembodiments, any suitable number of electrode pairs can be used.

In various embodiments, the layout of CB 66 can be designed in variousways to have the desired capacitances (71 and 73) between currentsources 55 and 57 and ground 62. The desired capacitances are typicallyon the order of 50 pF. Such a capacitance can be achieved, for example,by controlling parameters such as (i) the geometrical distance betweeneach current source and the nearest ground line, (ii) the thicknessand/or composition of a dielectric layer that separates between a groundlayer and a copper layer on which the current sources are fabricated.

In some embodiments, ASIC 60 is configured to hold at least a predefinedthreshold and, by comparing between the measured unipolar voltage andthe predefined threshold, to assess the physical contact between tissue33 and each electrode among electrodes 52, 54, 56 and 58. If, for agiven electrode, the value of the unipolar voltage is larger than thepredefined threshold, ASIC 60 may output an indication of physicalcontact between tissue 33 and the given electrode. Similarly, if thevalue of the unipolar voltage is equal to or smaller than the predefinedthreshold, ASIC 60 may output an indication of physical contact betweenthe given electrode and blood 43.

Additionally or alternatively, processor 20 is configured to hold thepredefined threshold, and, based on the value of the unipolar voltagemeasured by ASIC 60 on each electrode among electrodes 52, 54, 56 and58, to output an indication of whether there is physical contact betweeneach of the electrodes and tissue 33.

In the context of the present invention and in the claims, the term“electronic circuit” refers to any device configured to measure thesingle-ended voltage relative to ground 62 that is formed on at leastone of the electrodes in the pair in response to the electrical current,and, based on the measured voltage, to assess the physical contactbetween at least one of the respective electrodes and tissue 33. In anembodiment, the voltage measurement and the assessment of the physicalcontact may be carried out using a single device, such as ASIC 60 ofFIG. 2 or processor 20 of FIG. 1 . In another embodiment, the voltagemeasurement may be carried out using one device, e.g., ASIC 60, and theassessment of the physical contact may be carried out using anotherdevice, for example by processor 20 that receives the voltagemeasurements from ASIC 60. Note that in this embodiment, the term“electronic circuit” refers to a combination of ASIC 60 and processor20. In an alternative embodiment, system 10 may comprise any othersuitable configuration for measuring the voltage from the electrodes,and for assessing the physical contact based on the measured voltages.

In an embodiment, processor 20 is further configured to display, e.g.,in image 42, distal tip 13 overlaid on the image of heart 40, such thatsome of the electrodes may be in physical contact with tissue 33 and theremaining electrodes are in physical contact solely with blood 43.

Although the embodiments described herein mainly address cardiacablation and EP mapping procedures, the methods and systems describedherein can also be used in other applications.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art. Documents incorporated by reference inthe present patent application are to be considered an integral part ofthe application except that to the extent any terms are defined in theseincorporated documents in a manner that conflicts with the definitionsmade explicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

The invention claimed is:
 1. An apparatus, comprising: a current source,which is configured to flow an electrical current having a selectedfrequency between a pair of electrodes coupled to a medical probe; anelectronic circuit, which is configured to measure a single-endedvoltage relative to ground that is formed on at least one of theelectrodes in the pair in response to the electrical current, and, basedon the measured voltage, to assess physical contact between the at leastone of the electrodes and tissue; and a circuit board, which comprisesthe current source and the electronic circuit, and which comprises aphysical layout that produces, at the selected frequency of the currentsource, a predefined capacitance between the current source and ground,thereby establishing the ground as a reference ground for the currentsource, and wherein the electrodes and the current source are referencedto the reference ground established by the predefined capacitance, thusforming a reference for measurement of the single-ended voltage.
 2. Theapparatus according to claim 1, wherein the electronic circuit comprisesan application specific integrated circuit (ASIC).
 3. The apparatusaccording to claim 1, wherein the electrodes of the pair are fixed atopposite ends of a first section of the medical probe, and comprising anadditional current source, which is configured to flow an electricalcurrent between an additional pair of electrodes coupled to oppositeends of a second section of the medical probe, different from the firstsection, and wherein each of the additional electrodes is electricallycoupled to the electronic circuit so as to assess physical contactbetween at least one of the additional electrodes and tissue.
 4. Theapparatus according to claim 3, and comprising a processor, which isconfigured, based on the assessed physical contacts, to output whetherthere is physical contact between the tissue and at least one of (i) theelectrodes, and (ii) the additional electrodes.
 5. The apparatusaccording to claim 3, wherein the first and second sections are notoverlapped with one another.
 6. The apparatus according to claim 1,wherein the electronic circuit is configured to indicate that the atleast one of the electrodes is in physical contact with the tissue whenthe measured voltage is above a predefined threshold, and to indicatethat the at least one of the electrodes is not in physical contact withthe tissue when the measured voltage is below the predefined threshold.